Precoated steel plates, i.e. steel plates which are coated with paint before conducting a processing step (e.g. folding process), are widely used in the field of household appliances, automobiles and the like. The steel plates have advantages in atmosphere pollution, resource saving and labor saving, since a paint coating process after the processing is not necessary.
Contrary to the advantages, the paint to be precoated is required to have severe physical properties, especially two physical properties which are in conflict with each other (i.e. hardness and processability (e.g. folding ability)), in addition to normally required physical properties, such as adhesive properties, weather resistance, gloss and the like.
In order to comply with the required physical properties, especially hardness and processability, Japanese Kokoku Publication 62-21830 and Japanese Kokai Publication 59-8770 propose a polyester resin into which soft segments are introduced by using a monomer which is soft, such as 1,5-pentane diol, 1,6-hexane diol, sebacic acid and the like. Also, Japanese Kokai Publication 2-8242 proposes a polyester resin of which 10 to 80 mole % of a polyol component is 1,9-nonane diol, thus resulting in improvements in both hardness and processability.
However, the polyester resins proposed in the above mentioned publications adversely affect other properties when formulated into powder paint. For example, it is difficult to grind the polyester resins and they are not good for powder paint. When they are formulated into powder paint, it gives rise to the problems of poor storage stability, e.g. poor blocking resistance.